1. Field of the Technology
The present invention relates to an image forming apparatus based on electrophotographic technology such as a photocopier, printer and fax machine, particularly to an image forming apparatus capable of stable transfer of a toner image formed on an image carrier onto a transfer medium.
2. Description of Related Art
In the process of transferring a toner image onto the transfer medium, a spring back occurs when the trailing edge of the transfer medium passes through the transfer guide member to come into contact with the image carrier. The degree of spring back differs according to the type of the transfer medium and the conveying path before and after the transfer section. To ensure satisfactory transfer operation, a certain amount of curvature of concavo-convex structure is essential in the conveying path. Even if the guide member is placed close to the image carrier, a tough transfer medium such as a thick paper or heavy paper is very rigid, and the trailing edge of the transfer medium tends to hit the image carrier. In the example shown in FIGS. 4(a)-(b), spring back of the trailing edge occurs at 20 mm from the transfer area (FIG. 4 (a)) to cause an image failure wherein the toner image at the transfer area is scattered or misaligned by that impact shock (FIG. 4 (b)).
FIGS. 4(a)-(b) is a drawing representing the scattering of characters due to the spring back of the trailing edge of the transfer medium in the secondary transfer area of an intermediate transfer belt.
The following methods have been proposed to avoid the image failure of this type according to the conventional method: A flexible shielding plate such as a polyester film is bonded to the front edge of one guide member. When the transfer medium passes through the aforementioned guide member, the shielding plate presses the transfer medium elastically against the other guide member. When the transfer medium does not passes through it, the shielding plate comes in contact with the front edge of the other guide member, and the front edge of the aforementioned shielding plate is displaced in the conveying direction whereby spring back is prevented (e.g., Unexamined Japanese Patent Application Publication No. 10-123848). For a highly rigid transfer medium, a special mode can be selected by the special mode key, and a solenoid is turned on to perform such an operation that the front side of the transfer guide plate approaches the photoreceptor drum using the set screw on the back side as a fulcrum (e.g., Unexamined Japanese Patent Application Publication No. 5-289545). According to another proposal, only part of the transfer current value is changed.
However, when the flexible film member is bonded on the transfer guide member, the film member corresponding to the upper guide is pressed against the lower guide at all times. This method, therefore, cannot meet various types of sheets, and a problem occurs in the cased of thin paper. To be more specific, since thin paper is not sufficiently stiff, only one side of paper is bent conspicuously when a film having its one end formed to be protruded is pressed against the other guide, with the result that uneven conveyance occurs, and the front edge of paper cannot enter the nip of the transfer section. Since paper enters the nip in a bent form, the image is transferred on paper as it is tilted. Further, insufficient durability of the film can be a problem.
The method of approaching the transfer guide member to the image carrier (photoreceptor drum) will cause toner contamination.
There is a limit to the method of modifying and adjusting the transfer current value. To be more specific, although toner scattering and image misalignment can be reduced to some extent, when the current value is adjusted to the level that completely eliminates the possibility of toner scattering and image misalignment, an image failure will occur to the solid image over the entire surface or the half-tone image over the entire surface wherein there is caused a failure image of density change in that portion.
The object of the present invention is to provide an image forming apparatus capable of eliminating the possibility of an image failure caused by spring back, without using the film member, and without approaching the transfer guide member to the image carrier.